Been loosing sleep

Was just wondering. I have been working with applications of several kinds till now. I've recently been into an application that pulls data from the server and updates the server of its data. Like a channel, it always gets updated content each time you open the application. However, I've never developed an application that updates each client terminal as soon as the data on the central server is updated.

If I sound like I've slept bad, it is not entirely untrue. I am sitting past midnight and its not uncommon. However, I only want to ask you guys if you can offer me input as to how, say for instance, a stock broker's computer system gets automatically updated the second a scrip's price changes on the bourses. It would be foolhardly to assume the clients had timers which retrieved updates every few milliseconds. We have commonly seen software in the client/server model driven architecture wherein the client sends requests to the server for updates and the updates are exchanged. But this is a case wherein the server probably notifies clients by raising events. How is data exchange handled in this scenario. What could possibly be the underlying technology or set of technologies that make this miracle possible.

Finally, where can I find an overview of Electronic Data Interchange (EDI) for a-holes?